The invention relates to a pneumatic tire for vehicles, in particular a tire the architecture of which is optimized to facilitate fabrication while retaining high reliability and endurance.
It is common practice in the art, particularly for tires for passenger cars which must be capable of rolling at high speeds, to use a tire ply comprised of substantially circumferentially oriented cords for the purpose of reinforcing the stiffening hoop effect of the tire crown. This ply may be disposed radially outwardly of the other reinforcing plies customarily used in the crown. Such a tire ply is generally called a  less than  less than zero-degree ply greater than  greater than .
For use in typical passenger cars, the cords in the ply are generally based on Nylon. Because Nylon cord has a relatively low modulus of elasticity, it is usually laid at a high laying density, that is to say with a winding pitch the magnitude of which is close to the diameter of the cord itself.
In such classical configurations, the gap left between two adjacent cords, called the  less than  less than inter-cord space, i greater than  greater than , is small, often less than 0.4 "PHgr", where "PHgr" is the diameter of the cord. Most often,
i less than 0.25"PHgr".
Thus, for a cord of diameter 0.7 mm, the inter-cord space is typically on the order of 0.175 mm.
Accordingly, in order to obtain good bonding of the zero degree ply within the crown of the tire, it is necessary to apply around its cords an optimized layer of decoupling compound. This layer of decoupling compound must present high adherence to the cords and high strength, to enable said layer to resist shear stresses between adjacent cords. The composition of this layer of decoupling compound must also be such as to enable good adherence with the tread compounds, which latter are disposed radially outward of this layer of decoupling compound.
It has been found that it is difficult to achieve good adherence of the tread compounds with the zero degree ply comprised of substantially circumferentially oriented cords.
Definitions
In the following,  less than  less than cord greater than  greater than  is understood to mean both monofilaments and multifilaments, or assemblies such as cables, plied yams or any other type of equivalent assemblies, and this, regardless of the material or treatment of these cords, for example, surface treatment or coating or dipping to promote adhesion to the rubber.
 less than  less than Compound greater than  greater than  means a rubber mix having one or more base elastomer and additives selected in accordance with the properties of the compound desired for the zone of the tire it is used.
 less than  less than Layer of decoupling compound greater than  greater than  for a given reinforcing ply is understood to mean the rubber compound in contact with the reinforcing cords of the ply, adhering to these and filling the gaps between adjacent cords.
 less than  less than Contact greater than  greater than  between a cord and a layer of decoupling compound is understood to mean that at least part of the outer circumference of the cord is in intimate contact with the rubber compound constituting the decoupling compound.
 less than  less than Linear density greater than  greater than  applied to a cord is the weight in grams per 1000 meters of the cord, stated in units of  less than  less than tex greater than  greater than . The units used for the stress on a cord, or the modulus of elasticity of a cord are centi-newton per tex (cN/tex).
 less than  less than Winding pitch, p greater than  greater than , applied to a substantially circumferentially oriented cord which is spirally wound particularly in a large-diameter spiral having as its main axis the main axis of the tire, is the transverse distance between the cord axes of the cords of two adjacent loops in the spiral.  less than  less than  Laying density, d greater than  greater than , is the reciprocal of the winding pitch and thus corresponds to the number of loops of a spirally wound cord per unit axial length (along the main axis of the winding). Customarily, d is stated as the number of cords per decimeter (units of 1/dm), and p is stated in units of millimeters.
Thus p=100/d.
The fill density of reinforcing cords in a ply is characterized by the  less than  less than fill coefficient, FC greater than  greater than , defined as the ratio of the diameter of the cord to the laying distance:
FC="PHgr"/p,
where
"PHgr" is the diameter of the cord; and
p is the winding pitch of the cord.
Typically, this ratio is  greater than 0.6, and often it is  greater than 0.75.
The principal claimed matter of the invention relates to a tire, comprising a crown extended by two respective sidewalls and two respective beads, and a carcass which is anchored in said beads, said crown having the following components, disposed in the following order with progression radially from the interior to the exterior with respect to the main axis of the tire:
at least one reinforcing ply comprised of parallel cords disposed at an angle xcex1 in the range 10-75xc2x0 with respect to the circumferential direction;
at least one radially outermost ply having an axial width comprised of cords which are spirally wound wherewith the cords themselves are substantially oriented in the circumferential direction, said circumferentially oriented cords having a radially outward side of the cords and a radially inner side of the cords, and
a tread compound;
xe2x80x83wherein, in a substantial portion of the axial width of said at least one radially outermost ply, in any cross section perpendicular to the axis of the tire and intersecting a cord of said at least one radially outermost ply:
said tread compound has direct contact with the radially outward side of the cords of said at least one radially outermost ply,
there is a first decoupling layer comprising a decoupling compound different than the tread compound, said decoupling compound having direct contact with the radially inner side of the cords of said at least one radially outermost ply, the tread compound having contact with the decoupling compound in interfaces terminating axially on the cords of said at least one radially outermost ply; and
the one radially outermost ply has a fill coefficient FC xe2x89xa60.4, said fill coefficient being defined as
FC="PHgr"/p,
xe2x80x83where "PHgr" is the cord diameter of said cords and p is the winding pitch between the cord axes of neighboring said cords comprising the spiral winding.
It follows that the inter-cord space must be at least 1.5"PHgr"; thus for a cord of diameter 0.7 mm the inter-cord space must be at least 1.05 mm.
The direct application of the tread compound on the substantially circumferentially oriented cords facilitates the tire fabrication by limiting the number of components to be assembled. The wide winding pitch of these cords ensures excellent reliability and endurance of the adhesion between the tread and the rest of the crown, in that said adhesion is attributable principally to direct bonding between rubber compounds which are in mutual contact.
As substantial portion of the axial width of the radially outermost ply is meant at least 5% of this axial width.
Preferably, the fill coefficient of the radially outermost ply comprised of substantially circumferentially oriented cords is  less than 0.4 over the entire width of the radially outermost ply.
According to an advantageous variant embodiment, the fill coefficient of said radially outermost ply is lower in the central zone of the crown than in the lateral zones; thus the winding pitch of said circumferentially oriented cords is greater in the central zone of the crown. This enables a higher density of reinforcing components in the lateral regions (shoulder regions), which increases the resistance of the tire to high speed conditions.
According to a second variant embodiment, the fill coefficient of the radially outermost ply comprised of substantially circumferentially oriented cords is higher in the central zone of the crown than in the lateral zones; thus the density of reinforcing components is higher in said central region, which enables a flatter transverse profile of the tire, thereby improving tire performance in vehicle handling.
According to an other embodiment, the tread comprises two compounds, a first tread compound intended to come into contact with the road and an underlayer compound, different from the first tread compound, disposed radially inwardly of said first tread compound. In this embodiment, the underlayer compound is in direct contact with the said substantially circumferentially oriented cords of the radially outermost ply.
Such underlayers are known in the art. Compositions of such underlayers are intended to improve various aspects of performance, such as tire wear or the cornering stiffness of the tire. However, the optimization tends to come at the cost of inferior adherence to a cord structure and/or lower strength parameters, when compared to classical tread compositions applied by calendering.
Thus the invention offers the possibility of products with improved performance.
The substantially circumferentially oriented cords which are spirally wound develop a stress at 3% deformation greater than 12 cN/tex, and preferably than 20 cN/tex. Thus they offer a high modulus of elasticity at appreciable deformations, thereby allowing the ply of which they are comprised to perform its functions well, and in particular the conferring of strength to the crown during operation at high speed.
The said circumferentially oriented cords may also have an initial modulus of elasticity less than 900 cN/tex, and preferably less than 800 cN/tex. The advantage of the low initial modulus is a more comfortable ride and lower tire coast-by noise, during operation at low speed.
Such a cord may be a hybrid cord or yam comprised of at least one Nylon cord or yarn and at least one aramid cord or yam.